docs: def constants_pyx():
      """generate CONST = ZMQ_CONST and __all__ for constants.pxi"""
      all_lines = []
      assign_lines = []
      for name in all_names:
          if name == "NULL":
              # avoid conflict with NULL in Cython
              assign_lines.append("globals()['NULL'] = ZMQ_NULL")
          else:
              assign_lines.append('{0} = ZMQ_{0}'.format(name))
          all_lines.append('  "{0}",'.format(name))
      return dict(ASSIGNMENTS='\n'.join(assign_lines), ALL='\n'.join(all_lines))